1. Field of the Invention
This invention relates to a focus adjustment device which is capable of detecting a focusing state of an optical system by considering plural parts of a picture plane.
2. Description of the Related Art
It is known that one type of focus detection device for cameras is arranged to divide the exit pupil of a photo-taking lens and to discriminate focusing states by sighting displacements of relative positions of two images formed by light fluxes passing through the divided exit pupil areas. For a focus detection device of this type, a secondary image forming method has been disclosed, among others, in Japanese Laid-Open patent applications No. SHO 55-118019 and No. SHO 55-155331. In accordance with this method, aerial images formed on a predetermined focal plane (equivalent to a film surface) by two secondary image forming optical systems arranged in parallel are guided to two sensor surfaces and any displacement in the relative positions of the two images is detected.
FIG. 14 of the accompanying drawings shows in outline the secondary image forming type focus detection device. A field lens 43 is arranged to have an optical axis 42 in common with the photo-taking lens 41 the focus of which is to be detected. Two secondary image forming lenses 44a and 44b are disposed behind the field lens 43 and symmetrically with respect to the optical axis 42. Photo-electric conversion element arrays 45a and 45b are disposed behind the lenses 44a and 44b, respectively. The field lens 43 forms images of the exit pupil of the photo-taking lens 41 approximately on the pupil planes of the secondary image forming lenses 44a and 44b. When the aerial image formed near to the field lens 43 is imaged again by the secondary image forming lenses 44a and 44b on the surfaces of the photo-electric conversion element arrays 45a and 45b, the positions of the two images formed on the photo-electric conversion element arrays 45a and 45b are varied according to the displacement of the position of the aerial image in the direction of the optical axis. FIGS. 15(A), 15(B) and 15(C) show this condition. Of these figures, FIG. 15(A) shows an in-focus state. In this instance, each of the two images is located in the middle part of each of the photo-electric conversion element arrays 45a and 45b. FIG. 15(B) shows a far-focus state. In that instance, each of the two images moves in the direction from the optical axis 42. FIG. 15(C) shows a near-focus state, in which each of the two images moves in the direction of approaching the optical axis 42. The luminous intensity distribution of the two images is photo-electrically converted. Then, the discrepancy in relative position between the two images is detected to find a focusing state of the photo-taking lens 41.
A method for processing photo-electric conversion signals output from the above-stated photoelectric conversion element arrays 45a and 45b has been disclosed in Japanese Laid-Open Patent Application No. SHO 58-142306, U.S. Pat. No. 4,333,007, etc.
Further, a focus detection device which is arranged to be capable of detecting focus not only at a central part of a distance measuring picture plane but also at some other part of the picture plane by using a plurality of focus detection devices of the above-stated kind has been disclosed in Japanese Patent Application No. SHO 62-279835.
FIG. 16 shows in an oblique view the optical system of the above-stated focus detection device. Referring to FIG. 16, light fluxes passing through the two exit pupils 71a of a photo-taking lens 71 are field-limited by a field mask 72. The field-limited light fluxes then pass through different split parts 73a to 73e of a field lens 73, and then are imaged on different line sensors 75a to 75e of a single line sensor 75 by different parts 74a to 74e of a secondary optical system 74. Image data obtained through photo electric conversion by the line sensor 75 is subjected to a computing operation which is performed in a known manner to detect focus.
FIG. 17 shows, as another example, the optical system of a focal detection device which is also arranged to perform focus detection at a plurality of points. Referring to FIG. 17, light fluxes passing through the two exit pupils 80a of a photo-taking lens 80 are field-limited by a field mask 81. The field-limited light fluxes are allowed to pass through different parts of one and the same field lens 82. After that, the light fluxes are imaged on different sensor arrays SAA1, SAB1, . . . , SAA3 and SAB3 by image re-forming lenses 83 and 84. These images are photo-electrically converted and then computed in a known manner to detect focusing states obtained at a plurality of distance measuring points corresponding to the respective sensors.
When detecting focus at a plurality of distance measuring points, it is important to automatically select and determine a distance measurement result obtained at one of the distance measuring points based on which result the focus should be adjusted. To meet this requirement, one method for selecting an optimum distance measuring point has been proposed. According to the method proposed, focus adjustment is made on the basis of the output of a sensor in which a distance measurement result indicates the nearest distance. In some cases, however, the photographer wishes to take a picture of an object which is not located at the nearest distance. The above method, therefore, has presented a problem that it sometimes fails to perform focus adjustment as desired as it always selects the nearest distance.